Evolution of flavonols during Merlot winemaking processes

Abstract

Aim: The phenomenon of quercetin precipitation in wine (flanovol haze), has been manifested for many years in several wine-producing regions, such as Italy, Australia, and New Zealand (Gambuti et al., 2020; Lanati, Marchi, & Cascio, 2014; Somers & Ziemelis, 1985). Due to the limited information related to the quercetin aglycone behavior and its precursors during wine production in New Zealand, this study aims to monitor the evolution of flavonols and other polyphenols during the commercial fermentation of Merlot grapes, using different fermentation conditions, and vineyard treatments.

Methods: Various trials evaluating sun exposure, winemaking practices, and winemaking process management were undertaken using Merlot grapes, commercial yeast cultures, potassium metabisulphite (20 g/hL), and nutrient supplementation with DYNASTART®-LAFFORT at 20 g/hL. Samples were taken through the winemaking stages, and the polyphenols were quantified using a reversed-phase HPLC method (Garrido-Bañuelos et al., 2019; Peng et al. 2002).

Results: Grapes with elevated amounts of flavonols glycosides produced wines with higher levels of flavonol glycosides and quercetin. Wines made from grapes with greater sun exposure ended up with more flavonol glycosides (89 mg/L) and quercetin (16 mg/L) than the wines elaborated from less exposed grapes (47 mg/L and 9.4 mg/L, respectively). Certain winemaking practices showed differences in quercetin content, for example using small fermentation (250 kg) (12 mg/L), and large fermentation (five tonnes) (28 mg/L). The data also indicates that tannins and total anthocyanins were present at 786 mg/L and 156 mg/L, respectively, for small-scale ferments, and at 888 mg/L and 363 mg/L, respectively, for large-scale ferments. In evaluating the winemaking process management, the ferment pumped over (largest fermentation volume) exhibited flavonol glycosides and quercetin at the highest concentration (91 mg/L and 20 mg/L, respectively), compared to the remaining treatments. PMS, enzyme, and PMS plus enzyme, additions lowered the concentration of the flavonols glycosides at the end of the winemaking process (37 mg/L, 42 mg/L, and 43 mg/L, respectively). It was seem that the PMS plus enzyme (15.6 mg/L) increase quecetin in wine when compared to the control, no additions, (12.6 mg/L). The wines treated with enzyme, PMS, and PMS plus enzyme, also had lower concentrations of anthocyanins (215 mg/L, 233 mg/L, and 238 mg/L, respectively) than the control (291 mg/L). 

Conclusions

The study confirmed past research on the role of sun exposure in the formation of flavonols in Merlot grapes and wines. Fermentation size can improve the extraction of polyphenols into wine, and the enzyme additions can promote the hydrolysis of flavonol glycosides. In considering winemaking practices to lower flavonol content, the impact on remaining wine phenolics, of importance to wine colour and mouthfeel, also needs to be carefully evaluated.